In medical applications, nuclear magnetic reasonance (NMR) can indicate variations in the distribution of atomic substances in slices or volumes of interest within a patient. Such variations can be displayed in a way similar to the distributions provided by a computerized tomography system. In a nuclear magnetic resonance examination, magnetic and rf fields rather than x-radiation scan the body. Resonances caused by these fields are detected as induced signals in one or more detector coil systems. The outputs from these coils are then stored and analyzed so that NMR distributions can be displayed.
Techniques for producing these images are well known in the art and disclosed in various printed publications and U.S. patents. Several proposals for apparatus to utilize these procedures are embodied, for example, in U.S. Pat. Nos. 4,454,474 to Young, 4,384,255 to Young et al, and 4,379,262 to Young.
The techniques disclosed in the above mentioned prior art patents involve selection of a planar slice of interest in the body and application of a strong magnetic field gradient in a direction perpendicular to the slice. This field is perturbed in a perpendicular direction in the plane of the slice. The direction of the perturbation is continuously varied by a procedure documented in the literature.
The effect of this perturbation is to introduce a dispersion in nuclear resonance frequencies which return to their original unperturbed state in ways characteristic of the structure within the slice of interest. Repetition of this procedure for different directions can give many signals for each slice of interest which are then used to construct cross-sectional images descriptive of the internal structure of the patient slice.
The radio frequency perturbation excites the nucleii by realigning the macroscopic magnetization or magnetic moment within the cross-section of interest. This radio frequency energization is performed at the Larmor frequency. This frequency is related to a constant descriptive of the nucleii making up the region of interest and the magnetic field gradient imposed during perturbation.
Experience in NMR imaging indicates that the scanning times can be reduced and spacial resolution of NMR images can be increased by increasing this field to higher levels. Since the Larmor frequency of a given nucleii is directly proportional to the field strength, this increase in field strength must be accompanied by higher frequencies for RF energization. In the prior art this energization was accomplished with suitably designed energization coils which generate perturbation fields and in some instances are also used for detecting signals caused by resonances set up within the region of interest.
Transmission and reception of radio frequency signals for NMR imaging requires a resonant radiating structure, often called an rf coil, meeting several criteria. The resonant point of the structure must be high enough to allow proper tuning at the frequency of interest and the structure must have sufficiently high "Q" to provide good signal to noise performance in the receive mode.
Generally, in small volume nuclear magnetic resonance systems are unbalanced feed system and coil configuration is used. A simple reactive element is used as an impedance matching component and a second reactive element is used in parallel, with the coil structure to tune the coil to an appropriate frequency.
For large volume nuclear magnetic resonance applications, however, such as a head imaging system a balanced coil system is preferred. This is preferrable since under sample loading the coil system will be less influenced than an asymmetrical system.
As the frequency of operation is raised, however, the effectiveness of a symmetrical matching system is limited by a variety of factors. The reactive components become unmanageably small and are also subjected to extremely high peak voltages. The stray capacitance of the RF coil network eventually makes it impossible to match the network to a useful impedence. In addition to problems in achieving proper energization frequencies, use of larger RF coils creates problems in achieving a uniform magnetic field over the region to be perturbed.
Various prior art proposals to provide new and improved RF energization and detection coils are discussed in the literature. A publication entitled "Slotted Tube Resonator: A new NMR probe head at high observing frequencies" by Schneider and Dullenkopf discusses a resonator for use at high frequencies. This work was the first of a number of similar prior art publications discussing NMR resonator structures. Much of this work, however, has been conducted with extremely small dimensional structures which do not encounter the difficulties encountered when imaging a cross-section of a head. The task of converting a resonator coil for use in small structure analysis into a device suitable for NMR medical imaging is not a straight forward extension of this prior work.